Certain infrared (IR) detection systems are comprised of numerous optical components designed to reflect and transmit light in an 8 to 12 micrometer wavelength (.mu.m) range. When these systems are mounted in various vehicles for military and space applications, they must be provided with optical transmission through their enclosures and therefore require protective housings which also transmit light in the 8 to 12 .mu.m range. These systems and their housings are frequently required to operate at temperatures which range from a -65.degree. to 165.degree. F. in rain, sleet, snow, wind, dust and sand. Protection of these systems becomes even more important if they are used in high-speed aircraft where the detrimental effects of these environmental conditions are magnified. These conditions produce abrasion and corrosion and will degrade the performance of the optical window by erosion and by a chemical oxidation process. This erosion and oxidation reduces ultimately the usable lifetime of the window.
In the past, forward looking IR (FLIR) detection systems have been mounted in enclosures having IR windows fabricated from germanium, zinc sulfide, and zinc selenide. Germanium is frequently preferred because of its transmissive characteristics, physical characteristics and relatively low processing cost. However, the world supply of germanium is limited and therefore windows of this material must possess an extended lifetime in severe environments.
Germanium IR windows and other optical components such as the silver mirrors are attacked by the environmental conditions in which FLIR systems operate. These environmental conditions cause the transmissive character of the optical elements to deteriorate and necessitates a periodic replacement schedule which, depending upon the use, may be as often as every month. As a consequence of this replacement and maintenance necessity, the reliability of the system is decreased and the cost of these systems is measurably increased.
Zinc sulfide is the material of choice for windows used as outer elements in IR seeker systems employed on high-speed aircraft. These windows deteriorate by erosion caused by dust, sand, and rain which impinge upon, and ultimately abrade, the surface of such windows. This phenomenon results in decreased lifetimes of the windows and increased maintenance costs.
Various attempts to protect IR windows from the deleterious effects of the environments in which they are exposed have been unsuccessful. Coatings applied to IR window materials either tend to preclude IR transmission or fail to protect the crystals which form such material. The most relevant prior art known to me would appear to be an article entitled "IR Laser Window Coatings by Plasma Polymerized Hydrocarbons" by J. M. Tibbitt et al., that was published in the Proceedings of the Fifth Conference on IR Laser Windows Materials, Las Vegas, Nev., Dec. 1-4, 1975. Plasma polymerized ethane (PPE) coatings applied to sodium chloride crystals were reported in this article to reduce the sensitivity of these crystals to moisture. However, sodium chloride crystals having PPE coatings prepared by me in accordance with the teachings of the above Tibbitt et al. article began to degrade because of moisture uptake within a relatively short period of time and exhibited altered IR transmissive spectra.
There are no coatings for germanium and zinc sulfide crystals known to me which are transmissive in the 8 to 12 .mu.m region of the light spectrum and which adequately protect against water vapor and/or condensates, corrosion and abrasion when exposed to these conditions and phenomenon for relatively long periods of time.